A linear voltage regulator accepts a poorly specified and sometimes fluctuating input voltage and provides a substantially constant output voltage at a desirable level. The output voltage is used as a supply voltage for other circuits and is substantially independent of an output current (i.e., a load current). The load current level may vary over time with substantially instantaneous transitions from one level to another level.
For example, the linear voltage regulator supplies power to one or more digital circuits within a computer system which may be on or off depending on processing requirements. Thus, the load current level may be relatively high in one clock cycle and relatively low in a following clock cycle. As the digital circuits continue to improve and operate at higher frequencies, the transitions between clock cycles become faster, thereby decreasing the transition time between load current levels.
One example of an LDO regulator is disclosed in U.S. Pat. No. 6,952,091, issued on Oct. 4, 2005. According to this patent, a series resistor (Rs) is inserted in the output current path to sense the output current (Io), as shown in FIG. 1. The voltage drop across this sense resistance (Rs) is proportional to the output current (Io) of the regulator, which is fed back to a current limiting circuit. The current limiting circuit controls the drive to the gate of the output transistor (Qo) to limit the current. This arrangement suffers, however, from the drawback that the sense resistor causes a voltage drop leading to an undesired increase in voltage dropout.
Another example of an LDO regulator is disclosed in U.S. Pat. No. 5,191,278, issued on Mar. 2, 1993. Referring to FIG. 2, which is described in that patent, there is shown an LDO linear regulator that maintains a constant output voltage Vout at point 100, regardless of the magnitude of load 110. Power source 120, which provides the input voltage Vin, may be any type of power supply as currently known in the art.
The LDO regulator provides two feedback voltages to summer 130. The current sense and feedback loop, represented by block 140, provides as its output a voltage Vi directly proportional to the current being drawn by load 110. The second input to summer 130 comes from a voltage sense and feedback loop, represented by block 150. Block 150 provides a voltage directly proportional to the difference between Vout and a fixed reference voltage. The output from summer 130 gates pass device 160, which essentially provides a resistance inversely proportional to the voltage applied at its gate. The net result is that when either or both of the current feedback and voltage feedback inputs to summer 130 increases, the voltage out of summer 130 increases and the resistance through pass device 160 decreases, thereby allowing an increased flow of current through the pass device which keeps Vout at its desired level.
The present invention, which is described below, provides an improved LDO regulator, characterized by its ability to regulate the output voltage by using a differential sense amplifier to measure a small change in the output voltage. Furthermore, as described below, the present invention does not require a sense resistor, placed in series with the output load, to sense the output current. In conventional LDO regulators, as shown in FIG. 1, a sense resistor is inserted in series with the output current path to sense the output current. Such conventional arrangement suffers from a drawback that the sense resistor causes a voltage drop leading to an undesired increase in voltage dropout.